1. Field of the Invention
The present invention relates to a gas sensor control apparatus.
2. Description of Related Art
Limiting-current type oxygen concentration sensors are in practical use in vehicle engines to measure oxygen concentration in exhaust gas. This type of oxygen concentration sensor includes a sensor element constituted of a solid electrolyte layer made of, for example, zirconia, and a pair of electrodes located on the opposite sides of the solid electrolyte layer. When a voltage is applied across the electrodes, the sensor element outputs a current signal having a value depending on oxygen concentration in exhaust gas.
The sensor element has to be heated to a predetermined temperature range (about 700-800 degrees C.), so that it is put in an active state in which the current signal can be outputted accurately, enabling detection of oxygen concentration or an A/F (air-fuel ratio). Generally, the sensor element is provided with an electrical heater. When a vehicle engine is started, the heater is energized to put the sensor element in the active state, and thereafter, the heater is energized as necessary to keep the sensor element in the active state.
To determine whether the sensor element has entered the active state, an activation judgment is performed. As one way of making such an activation judgment, it is known to measure an element impedance of the sensor element, and use the measured element impedance as a judgment parameter. For details, refer to Japanese patent Application Laid-open No. 2002-5882, for example. There is a certain correlation between the temperature of the sensor element and the element impedance. As the temperature of the sensor element increases, the element impedance decreases. Accordingly, it is possible to judge that the sensor element has entered the active state by detecting that the element impedance has fallen below a predetermined threshold (several tens of ohms, for example).
On the other hand, recently, with the aim of starting an air-fuel ratio feedback control as soon as a vehicle engine is started, attempts have been made to monitor a sensor output, and make a judgment that the sensor has entered the active state when the sensor output has converged within a predetermined normal range. However, the inventors of the present application have found that it takes a long time for the sensor output to converge within a predetermined normal range from the start of a vehicle engine, because moisture or organic matter adheres to the sensor element while the engine is stopped. This delays making a judgment that the sensor element has entered the active state.